Most first class mail isn't touched by human hands until the mail carrier puts it into your mailbox. I'm guessing that you could ship a machine-sortable as a pa

Most first class mail isn't touched by human hands until the mail carrier puts it into your mailbox. I'm guessing that you could ship a machine-sortable as a parcel with tracking if you brought it to the post office, but not when it's being mixed in with a thousand other machine-sortables in the out-going mail bag. You don't want to go through each machine-sortable letter to see if tracking is required. I suspect this is less about bureaucratic complexity and more about dealing with a corner case caused by operational efficiency. edit: Jacob on the consumerist thread explained it better than I did. "I suspect that letters (or flats) are processed through equipment that doesn't scan the barcode for the delivery confirmation system. Only if your letter is really a package and doesn't fit through the equipment that processes letters does it go through the package system which does scan the barcodes."